Mobile speaker phones for use within automobiles and other vehicles are subject to echo generation within the vehicle. That is, sounds generated by a loudspeaker of the phone can reverberate within the vehicle and be sensed by a microphone of the phone as an echo. To prevent the return of such echos to the far end user, echo cancellation techniques are often employed. To perform echo cancellation, a response function of the entire channel from the input of the loudspeaker to the output of the microphone is often generated. This channel is typically characterized as a series of three filters; namely, a loudspeaker filter, an echo filter, and a microphone filter. Knowing the input signal of the loudspeaker, the response function can be used to estimate the echos that will be present in the output signal of the microphone. These estimated echos can then be subtracted from the output signal of the microphone to significantly reduce the level of the echos therein.
In the past, the loudspeaker filter, the echo filter, and the microphone filter were all modeled as linear filters. It has since been found that loudspeakers, particularly small, less expensive loudspeakers, are more accurately modeled as nonlinear filters. During speaker phone operation within a vehicle, the echo response of the vehicle chamber will be continuously changing. Thus, it is necessary to adapt the response function used by the echo cancellation functionality on line (i.e., during communication operations). It is difficult, however, to adapt a nonlinear filter response on line. For this reason, the nonlinear response function of the loudspeaker, which does not change much during system operation, can be determined offline and then combined with the adapting linear response function of the echo and microphone on line. Past methods for determining the nonlinear response function of a loudspeaker have been complicated and have required expensive test set ups (e.g., an anechoic chamber set up) to perform. There is therefore a need for relatively simple and inexpensive methods and structures for determining nonlinear loudspeaker response functions.